Evaluation of the fraction of delayed photoneutrons for TMSR-SF1

The 10 MW_th solid-fueled thorium molten salt reactor (TMSR-SF1) is a FLiBe salt-cooled pebble bed reactor to be deployed in 5-10 years, designed by the TMSR group. Due to a large amount of beryllium in the core, the photoneutrons are produced via (γ, n) reactions.Some of them are generated a long time after the fission event and therefore are considered as delayed neutrons. In this paper, we redefine the effective delayed neutrons into two fractions: the delayed fission neutron fraction and the delayed photoneutron fraction. With some reasonable assumptions, the inner product method and the k-ratio method are adopted for studying the effective delayed photoneutron fraction. In the k-ratio method, the Monte Carlo code MCNP6 is used to evaluate the effective photoneutron fraction as the ratio between the multiplication factors with and without contribution of the delayed neutrons and photoneutrons. In the inner product method, with the Monte Carlo and deterministic codes together, we use the adjoint neutron flux as a weighting function for the neutrons and photoneutrons generated in the core. Results of the two methods agree well with each other, but the k-ratio method requires much more computing time for the same precision. (authors)